As is well known, drilling fluid is utilized in well-drilling operations for a number of basic purposes. One purpose is to cool and lubricate the bit and the string. Another is to carry up to the surface the chips of rock, etc. which are produced as a result of the drilling operation. A third purpose is to deposit a tough, impermeable filter cake against the sides of the bore-hole and thus block up the tiny interstices in the rock wall through which the fluid phase of the drilling fluid could escape. A fourth is to control the pressure in the well in order to prevent "blow-out" due to a higher pressure in the formation than in the well. A fifth is for control of corrosion of the string and bit. A sixth purpose is to stabilize the well bore chemistry. A seventh is to buoyantly support the drill string.
Due to geothermal heat in the surrounding formations, the temperature of the drilling fluids can rise as high as 600.degree. F. or more. The pressure of the drilling fluid depends, of course, upon the depth of the bore-hole. For very deep wells, the pressure placed upon the drilling fluid at the bottom of the well can be as high as 20,000 psi or more.
It is very important for the drilling fluid not only to "cake" and seal against the sides of the bore-hole at locations where the liquid portion of the drilling fluid tends to filter away into the surrounding rock formation, but also to provide a cake layer which will resist erosion by the flowing of the drilling fluid upwardly in contact with the bore-hole walls, and will lessen friction between the drill string and the bore-hole walls.
The pertinent prior art contains patents which are adapted to test the caking quality of a drilling fluid, but none of these is capable of doing so under conditions in which the fluid itself flows past an element simulating permeability and pore size of a rock formation. Moreover, the prior art methods and apparatuses do not have the capability of carrying out this testing under conditions of high pressure and temperature such as those found under actual drilling conditions.
U.S. Pat. No. 2,217,175, Ledbetter, issued Oct. 8, 1940, relates to a method and apparatus for testing drilling fluids, in which the caking quality of the fluid has an effect upon the length of time taken for the fluid phase of the slurry, after contact with a filter paper, to percolate radially outwardly along the filter paper.
U.S. Pat. No. 2,646,678, Standing, issued July 28, 1953, relates to a device adapted to determine filter loss in drilling fluids under static conditions, and is not capable of taking into account the erosion rate resulting from drilling fluid flowing past a caked layer on the sides of a bore-hole.
U.S. Pat. No. 2,830,266, Southwick, issued Apr. 8, 1958, is of general interest in that it discloses a method and apparatus for filtering drilling fluid within a bore-hole, by obtaining a specimen of the filter cake and then measuring the thickness and/or the electrical properties of the filter cake.
U.S. Pat. No. 3,055,208, Gallus, issued Sept. 25, 1962, discloses a filter adapted to measure the rate and extent to which fluids are lost to sub-surface strata from drilling fluids, under conditions up to 1,000 psi.
U.S. Pat. No. 3,172,286, Grubb, issued Mar. 9, 1965, also discloses apparatus intended to test certain characteristics of drilling fluids.